Interconnects for use in integrated circuits are generally made of aluminum but aluminum is unsatisfactory in high speed semiconductor devices. The resistivity of aluminum as well as its electromigration limits and restrict its use in such applications. The present invention contemplates use of high purity copper as interconnects for use in integrated circuits which have requirements of line width about 0.18 xcexcm or less and are to be used in semiconductor devices at 1200 Mhz clock speeds. The interconnect lines are created by sputtering high purity copper from sputtering target assemblies.
To produce very small line widths, as described, we have determined that the sputtering target should be of high purity, i.e. at least 99.999 wt .% purity, (referred to as 5N copper) and preferably 99.9999 wt. % purity (referred to as 6N copper). Purity is important to maintain the low resistivity of the copper line so that high speed goals can be achieved. Moreover, purity may also influence electromigration resistance.
We have also determined that it is also desirable that the sputtering target have a fine and substantially uniform grain size, preferably about 50 xcexcm or less. The fine grain size aids in achieving a uniform film thickness during deposition, allows the target to sputter faster, and seems to result in fewer particle problems on the substrate, i.e. wafer. It is also important in accordance with the invention for the target to be diffusion bonded to a light weight, high strength backing plate. As targets become larger, particularly those designed for 300 mm diameter silicon wafers, the weight of the target becomes a significant handling factor. Technically, it would be possible to decrease the thickness of the sputtering target surface because only a part of the target is consumed. However, in order to take advantage of this capability, it is necessary to bond a thinner target blank onto a lightweight backing plate provided the thinness of the sputtering target surface does not allow the target assembly to warp during use. Warping of the target can lead to inconsistent deposition as well as particle generation. Therefore, it is important that the backing plate be strong and stiff in addition to being lightweight. The present invention achieves these objectives by providing a sputtering target assembly comprising high purity copper target diffusion bonded to a precipitation hardened aluminum backing plate. By use of diffusion bonding to join the target to the backing plate it is possible to avoid the need for soldering which is undesirable because temperatures required for sputtering often are sufficiently high to melt the solder bond of the targets and, moreover, the heat generated has potential to continue grain growth in the target after long periods of use.
In accordance with the invention there is provided a sputtering target assembly comprising a high purity copper target, a precipitation hardened aluminum alloy backing plate and an intermediate layer of CuCr diffusion bonded to the target and backing plate. Desirably, the aluminum is in the fully hard T6 condition and the sputtering target comprises copper of a purity of at least about 99.999 wt. %. The sputtering target also contains a micro-alloy grain stabilizers comprising at least one of Ag, Sn, Te, In, Mg, B, Bi, Sb, and P. The stabilizer is preferably present in an amount of about 0.3 ppm to 10 pmm. The intermediate CuCr comprises copper and about 0.5 to 1.5 wt. % Cr, preferably about 1%. The high purity copper target has a substantially uniform grain size of not more than about 50 xcexcm.
In another embodiment of the invention there is provided a method of making a sputtering target assembly which comprises providing high purity copper target of at least about 99.999 wt.% purity. Preparing a master alloy of copper and not more than about 10 ppm of at least one of the micro-alloy grain stabilizers described previously, preparing molten combination of high purity copper and the master alloy and solidifying the molten combination to produce a cast billet; hot deforming the cast billet for a total of at least about 50% deformation on each of the axes and then rapidly quenching the deformed billet, preferably in water; frictionless forging the quenched billet at elevated temperature to about 70% of the starting length of the billet and rapidly quenching preferably in water; cold rolling to a total of about 90% deformation; producing an aluminum alloy backing plate having a precladding surface of CuCr diffusion bonded thereto; diffusion bonding the high purity copper target to the preclad CuCr surface; and precipitation hardening the aluminum alloy backing plate to the fully hard T6 condition.
Preferably the master alloy is prepared by combining a major amount of high purity copper with a minor amount of at least one of the micro-alloy stabilizers Ag, Sn, Te, In, Mg, B, Bi, Sb, and P, melting the combination and casting to produce a master alloy. In the preferred embodiment the master alloy is formed by. combining high purity copper with at least one of the micro alloy stabilizers in a ratio of about 1,000 to 1.
The backing plate having a precladding surface of CuCr diffusion bonded thereto is preferably produced by a process comprising embedding an alloy of Cu and Cr in an aluminum or aluminum alloy envelope and e-beam welding the envelope closed in a vacuum environment; heat treating the enclosed envelope and forging to bring the CuCr into intimate contact with the aluminum alloy, and then heat treating to diffusion anneal and solutionize the hardening elements in the aluminum alloy, quenching and thereafter removing the aluminum alloy envelope to expose the CuCr surface and precipitation harden the aluminum alloy to a fully hard T6 condition.
Sputtering targets in accordance with the invention may be used to produce an interconnect for use in an integrated circuit, having a width 0.18 microns or less and comprising copper of at least about 99.999 wt.% purity, preferably copper of at least about 99.9999 wt.% purity.